1. Field of the Invention
The present invention relates to a process for preparing aromatic hydrocarbons and, more particularly, to a process for efficiently preparing hydrocarbons rich in benzene, toluene, xylene and so on, useful for a base material of high octane number gasoline or for petrochemical raw materials, from hydrocarbons having from 6 to 12 carbon atoms.
2. Description of Related Art
As naphtha containing more volatile hydrocarbons, particularly light naphtha, has a limited field of uses, demands have been made to develop technology of efficiently converting the naphtha into aromatic hydrocarbons having a high content of benzene, toluene, xylene and so on, which have more value than the naphtha. Likewise, it is considered to be desirable to provide improved technology for reforming a variety of comparably lighter non-aromatic hydrocarbon fractions including light naphtha and gasoline fractions, into aromatic hydrocarbons rich in benzene, toluene and xylene.
Representative of a process for reforming naphtha using a platinum-deposited catalyst is a process for reforming non-aromatic hydrocarbons into aromatic hydrocarbons. Heretofore, however, a large amount of hydrogen has been supplied to the reaction system, together with a raw material for the reaction, in order to lengthen the life of the catalyst. Unless hydrogen is charged in a large amount, coke may deposit causing deactivation of the catalyst.
This process is effective to prevent deactivation of the catalyst to be used, however it suffers from the disadvantages that hydrogen to be used in a large amount is relatively expensive and hydrogenolysis of the hydrocarbon to be used is likely to occur, thereby incurring the risk of reducing selectivity to useful aromatic compounds.
In order to maintain catalytic activity and provide highly improved selectivity to useful aromatic compounds, for example. U.S. Pat. No. 4,416,806 proposes a catalyst with a metal belonging to the group VIII of the periodic table, such as platinum, deposited on a so-called macroporous zeolite such as an L-type zeolite or the like.
As is apparent from the examples of this U.S. Pat. No. 4,416,806, the catalyst used therein also requires hydrogen at a ratio to hydrocarbons of 0.2 to 1 in order to improve selectivity and extend the life of the catalyst. It is to be noted, however, that although the catalyst proposed in this prior patent publication is claimed to be an extended life catalyst, the yield of the aromatic hydrocarbons when using this catalyst is reduced from 38% to 23% as the process time is extended from 5 hours to 77 hours. Hence, the life of catalyst to this extent cannot be said to be sufficiently long from the viewpoint of an industrially available process.
In order to use the catalyst on an industrial basis, the catalyst is required to have a life extended to a sufficiently long period of time and yet to provide a sufficiently high selectivity to useful aromatic hydrocarbons.